The absence of highly porous, uniformly nanoporous surfaces places limitations on handling, fractionating, extracting, recovering and concentrating aqueous and nonaqueous samples of chemicals and biological substances, both soluble and insoluble. For example, if the surface is not highly porous, then a process that depends on the surface porosity will be slowed down. A surface that is not nanoporous is limited in its ability to provide size separations or retentions involving dissolved nanosubstances, especially if the size of the pores is not uniform. The porosity, size and shape of a surface for one sample will not be suitable for all samples and processes, making control of these surface properties important. A surface that is non-polar or not highly hydrophilic is prone to causing adsorption losses of nanosubstances that are more or less non-polar from aqueous samples. Such adsorption can build up to clog the surface. A surface that is not highly nonpolar is limited in its ability to process nonaqueous samples. Every known bulk, porous surface is limited in one or more of these properties.
Commercially-obtainable, nonhydrogel membranes are commonly prepared by a phase separation process in many pore sizes. These membranes are always non-polar to some degree, associated with their strength and flexibility, and the pores when abundant in the nano regime always range in size. Losses of nanosubstances on such membranes either due to adsorption near, into or onto size-complementary pores, or via flow through the larger pores, are common. This is particularly true for smaller nanosubstances such as peptides and oligosaccharides. When a small nanosubstance is relatively pure and present in a low concentration in an aqueous sample, recovery may not great as other substances in a pure sample are lacking that ordinarily can block some of the adsorption sites, or fill the larger pores, to help recovery of the nanosubstance of interest when placed in contact with the membrane.
The detection of nanosubstances such as peptides and oligosaccharides by mass spectrometry is compromised by the performance of current porous surfaces for size-based purification as for desalting which generally is needed to get the best signals.
The surface problem also is present in size exclusion chromatography, where it has been not possible to fabricate a narrow pore size distribution in chromatographic media such as particles or monoliths. Thus, these columns for preparative size exclusion separations have extra wide diameters since the pores of interest are only a fraction of the overall range of pores that the columns provide.
Grafting and other treatments of gel surfaces also have been unsuccessful in reducing surface pores uniformly, reproducibly, and in a controlled fashion, since the starting pores are heterogeneous, and grafting anyway tends to give a heterogeneous surface. While the surface of a hydrogel has been toughened by ambient treatment with hydrogen peroxide, the permeability as desired was maintained or increased, not reduced with formation of uniform pores.
Known size exclusion techniques and products give a mixed-mechanism separation due to nonpolar or ionic sites on the packings, which complicates or interferes with separation. Adding a non-polar organic co-solvent or salt to an aqueous sample does not completely overcome the issues presented by the technology and introduces other problems.
Accordingly, there remains a need for surfaces which allow for size exclusion techniques for particles of small molecular size with little or no loss of sample due to sticking to the surface and which provide a high capacity for a sharp separation based on size. Once the size difference between small particles dips below two-fold, separation by size exclusion begins to fail with current chromatographic packings. There also is a need for plastic chromatographic surfaces which avoid or minimize the problem of micorpores that causes chromatographic band broadening and losses for small analytes. In other words, a method is needed that fills in or closes these pores, and in a uniform way.